feat: add redirectUser callback

[owencraston]

What

This PR enables this issue: https://github.com/MetaMask/accounts-planning/issues/35

This callback will be implemented in this extension PR.

For async transactions, we call this callback which will trigger a confirmation modal in the extension with the URL and the message as part of the UI. This will be used to instruct users to complete signings at the specified URL or app.

How

• add a redirectUser callback with the following structure

  redirectUser(
    snapId: string,
    url: string,
    message: string,
    method: string,
  ): Promise<void>;

• inside submitRequest, add the following block

    if (response.redirect?.message || response.redirect?.url) {
      const { message = '', url = '' } = response.redirect;
      await this.#callbacks.redirectUser(snapId, url, message, method);
    }

• In our case, these fields will be populated in the snap-simple-keyring dapp. I am currently in the process of making these changes here